Apple is aware of an iMessage issue caused by a specific series of unicode characters and we will make a fix available in a software update. Until the update is available, you can use these steps to re-open the Messages app.

● Ask Siri to "read unread messages."● Use Siri to reply to the malicious message. After you reply, you'll be able to open Messages again.● In Messages, swipe left to delete the entire thread. Or tap and hold the malicious message, tap More, and delete the message from the thread.